Electric cars or hybrid cars include at least one electric motor for driving the car and a rechargeable battery (accumulator), such as a lithium-ion battery, or a double layer capacitor arrangement, for supplying the at least one motor with electric power. The battery usually includes a plurality of battery modules that are connected in series, where the individual modules may be arranged distributed throughout the car and connected with each other by power lines.
The individual battery modules each provide a module supply voltage, where the supply voltage of the battery equals the sum of the module supply voltages. Due to variations in the manufacturing process of the individual modules the capacitances of the individual battery modules may be slightly different. Thus, some battery modules may be charged faster than others during a charging process in which the battery is charged, and some battery modules may be discharged faster than others during a discharging process, which is when the battery supplies power to the load. The individual battery modules should not be charged above an upper charging limit and should not be discharged below a lower charging limit. Thus, unless there is a charge balancing, the charging process would have to be stopped when one module reaches the upper limit, although other modules may not have been completely charged yet, and the discharging process would have to be stopped when one module reaches the lower limit, although other modules may not have been completely discharged yet. Different charging states of the individual modules could also result from a self-discharging of the modules.
“Charge balancing” means that electrical charge is selectively taken from individual modules or is selectively provided to individual modules in order to compensate for charge imbalances of the individual modules.
A known charge balancing circuit includes a transformer with a plurality of first windings each connected to one of the modules and a second winding connected between the load terminals of the battery. In this circuit, electrical charge can be taken from one of the modules and can be fed back into the battery, or charge can be taken from the battery and can be fed back into one of the battery modules. In this circuit, however, the battery voltage, which can be several 100V, such as 400V, is present across the second winding. This requires high voltage insulation means which can be difficult to implement and costly when the modules are distributed in the car.
There is a need for a power supply system with an efficient, flexible and easy to implement charge balancing.